The function of a track following servo system for longitudinal tape, such as magnetic tape, is to move a head laterally of the longitudinal tape to accurately follow the lateral movement of the tape, for example, during read/write operations of the head. If done accurately, the data tracks are written and read in straight lines along the longitudinal tape as the tape is driven in the longitudinal direction. With respect to magnetic tape, the data comprises parallel stripes recorded in the longitudinal direction of the magnetic tape. Servo tracks are prerecorded in the magnetic tape parallel to, and offset from, the expected data stripes. Typically, the lateral movement of the magnetic tape is constrained by flanges present on tape guides at either side of the head, such that the servo system causes the head to follow the data stripes in the presence of disturbances mainly created from limited lateral motion of the tape, called LTM (Lateral Tape Motion).
Servo systems often employ compound actuators to move the head laterally both for track following, and to shift from one servo track (or set of servo tracks) to another and to follow a different set of data stripes. A compound actuator, which comprises a coarse actuator and a fine actuator mounted on the coarse actuator, provides both a large working dynamic range and high bandwidth. The high bandwidth fine actuator typically has a limited range of travel to attain the high bandwidth, and, in the typical track following arrangement, with the fine actuator as the master and the coarse actuator as a slave to the movement of the fine actuator, if the fine actuator drifts to one side as the tape moves laterally, the coarse actuator follows (at a slower rate) the centerline of the movement of the fine actuator.
The flanges of the tape guides, such as rollers, limit the lateral motion of the tape, but may tend to flex the tape and to introduce debris accumulation of the flanges that impact the lifetime of the tape and in addition create undesirable dynamic effects.
Flangeless tape guides tend to solve the problems of the flanged tape guides, but, without being constrained, the longitudinal tape tends to rapidly shift from one side of the path to the other, and may run at one side of the path for only a short period. Thus, in an attempt to follow the tape from one side to the other, the coarse actuator, in following the centerline of the movement of the fine actuator, is required to move at its highest rate from side to side as the tape rapidly shifts. This motion tends to wear and shorten the life of the coarse actuator, and is a use of power by the coarse actuator.
The incorporated '403 application centers the coarse actuator at the midpoint of the lateral shift excursions rather than attempting to follow the fine actuator. Thus, the fine actuator follows the lateral shift excursion, while the coarse actuator remains at the midpoint.